RFC 3623 (rfc3623) - Page 2 of 18
Graceful OSPF Restart
Alternative Format: Original Text Document
RFC 3623 Graceful OSPF Restart November 2003
Table of Contents
1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Operation of Restarting Router . . . . . . . . . . . . . . . . 3
2.1. Entering Graceful Restart. . . . . . . . . . . . . . . . 4
2.2. When to Exit Graceful Restart. . . . . . . . . . . . . . 5
2.3. Actions on Exiting Graceful Restart. . . . . . . . . . . 6
3. Operation of Helper Neighbor . . . . . . . . . . . . . . . . . 7
3.1. Entering Helper Mode . . . . . . . . . . . . . . . . . . 7
3.2. Exiting Helper Mode. . . . . . . . . . . . . . . . . . . 8
4. Backward Compatibility . . . . . . . . . . . . . . . . . . . . 9
5. Unplanned Outages. . . . . . . . . . . . . . . . . . . . . . . 10
6. Interaction with Traffic Engineering . . . . . . . . . . . . . 11
7. Possible Future Work . . . . . . . . . . . . . . . . . . . . . 11
8. Intellectual Property Rights Notice. . . . . . . . . . . . . . 11
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
9.1. Normative References . . . . . . . . . . . . . . . . . . 11
9.2. Informative References . . . . . . . . . . . . . . . . . 11
A. Grace-LSA Format . . . . . . . . . . . . . . . . . . . . . . . 13
B. Configurable Parameters. . . . . . . . . . . . . . . . . . . . 15
Security Considerations. . . . . . . . . . . . . . . . . . . . . . 16
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17
Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 18
1. Overview
Today many Internet routers implement a separation of control and
forwarding functions. Certain processors are dedicated to control
and management tasks such as OSPF routing, while other processors
perform the data forwarding tasks. This separation creates the
possibility of maintaining a router's data forwarding capability
while the router's control software is restarted/reloaded. We call
such a possibility "graceful restart" or "non-stop forwarding".
The OSPF protocol presents a problem to graceful restart whereby,
under normal operation, OSPF intentionally routes around a restarting
router while it rebuilds its link-state database. OSPF avoids the
restarting router to minimize the possibility of routing loops and/or
black holes caused by lack of database synchronization. Avoidance is
accomplished by having the router's neighbors reissue their LSAs,
omitting links to the restarting router.
However, if (a) the network topology remains stable and (b) the
restarting router is able to keep its forwarding table(s) across the
restart, it would be safe to keep the restarting router on the
forwarding path. This memo documents an enhancement to OSPF that
makes such graceful restart possible, and automatically reverts back
Moy, et al. Standards Track